1. Field of the Invention
The present invention relates to second-order nonlinear glass material and a method for producing it. More specifically, the present invention relates to a SiO2 glass material having high second-order nonlinearity for a long period, and a method for producing it.
2. Description of the Related Art
Conventionally, there have been known optical functional elements such as optical switches, optical modulators, wavelength converter or the like, as an apparatus using second-order nonlinearity such as generation of secondary higher harmonics or the like. These are generally made by use of a crystalline material represented by LiNbO3.
In an optical system in optical communication, optical measurement or the like, an optical fiber made of a SiO2 glass material is used as a signal transmission line. Accordingly, considering connectivity with an optical fiber and cost performance, it is strongly required that the above-mentioned optical functional element is also made by use of a SiO2 glass material in place of a crystalline material such as LiNbO3.
However, a glass material does not inherently have second-order nonlinearity, because of inversion symmetry thereof. This is one of reasons why a glass material is used only for passive utility.
There is proposed a ultraviolet poling treatment method comprising subjecting a glass material to ultraviolet radiation and application of high electric field (poling) at the same time to afford second-order nonlinearity to the glass material. The method enables the glass material to express second-order nonlinearity that is comparable to LiNbO3 crystalline. Accordingly, it is expected that the method enables a glass material to be used as a material for optical functional elements.
However, the second-order nonlinearity expressed in a glass material by the ultraviolet poling treatment is attenuated to about 40% of the initial value only after about 280 days at room temperature. Namely, there is a problem that the lifetime of the second-order nonlinearity is very short. The reason may be that the second-order nonlinearity expressed by destroying inversion symmetry by ultraviolet poling treatment is alleviated to be in an original state by thermal energy. Considering the period of use of optical functional elements, second-order nonlinearity need to have a lifetime of at least 10 years.
Accordingly, the most important subject in application of second-order nonlinear glass material to optical functional elements or the like is to prolong the lifetime of the second-order nonlinearity induced by the ultraviolet poling treatment.
The object of the present invention is to provide second-order nonlinear glass material having second-order nonlinearity which is a sufficiently high and has a sufficiently long lifetime for a practical purpose, in use of the glass material for optical functional elements or the like.
The present invention relates to second-order nonlinear glass material wherein a part having second-order nonlinearity contains Ge, H and OH and has second-order nonlinear optical constant d of 1 pm/V or more.
As described above, the glass material of the present invention is characterized in having a structure that a part having second-order nonlinearity contains Ge, H and OH. Namely, the glass material of the present invention has a structure that unpaired electron exists in a defect which is a cause of generation of second-order nonlinearity (called GeExe2x80x2 center) is terminated with H or the like. Such a structure can prevent GeExe2x80x2 center from being alleviated to be in an original state by thermal energy, and thus second-order nonlinearity can continue to be expressed for a long time. Furthermore, since the glass material has a large second-order nonlinear optical constant d of more than 1 pm/V or more, it can be applied to optical functional elements or the like.
Preferably, a period in which the second-order nonlinear optical constant d is decreased to 1/e of an initial value at room temperature in the above-mentioned second-order nonlinear glass material, is 15 years or more.
The above-mentioned glass material having second-order nonlinearity of long lifetime is suitable as a material for optical functional elements to be used for a long period.
The present invention also provides a method for producing second-order nonlinear glass material comprising treating a porous glass material containing Ge with hydrogen, sintering it and subjecting it to a ultraviolet poling treatment.
As described above, when hydrogen such as H is previously introduced in a glass material by a hydrogen treatment, unpaired electrons in a GeExe2x80x2 center generated by the ultraviolet poling treatment can be terminated with hydrogen cation. As a result, the GeExe2x80x2 center is prevented from being alleviated by thermal energy to be in an original state, and a glass material having second-order nonlinearity for a long period can be provided.
Preferably, the hydrogen treatment is conducted at a temperature of 800xc2x0 C. or less in the above-mentioned method for producing second-order nonlinear glass material.
As described above, when the hydrogen treatment is conducted at a temperature of 800xc2x0 C. or less as described above, Ge as a source of the GeExe2x80x2 center can be prevented from being volatilized as GeO. As a result, a sufficient amount of GeExe2x80x2 centers can be generated by the ultraviolet poling treatment, and a glass material having high second-order nonlinearity can be provided.
According to the present invention, the glass material is produced by conducting the hydrogen treatment before the ultraviolet poling treatment, and thus it is possible to last second-order nonlinearity of the resulting glass material for a long time. As a result, second-order nonlinear glass can be applied to optical functional elements such as optical switches.